1. Field of the Invention
The present invention generally relates to a semiconductor manufacturing apparatus. More particularly, the present invention relates to an exposure apparatus of photolithographic equipment.
2. Description of the Related Art
Photolithography is used to form a fine circuit pattern on a wafer. A photolithographic process generally includes cleaning the surface of a wafer, coating the surface with photoresist, aligning the wafer with a mask or reticle, exposing the layer of photoresist to light directed through the mask or reticle, and developing the exposed layer of photoresist. The alignment and exposure processes are carried out using an exposure apparatus.
In addition to a mask or reticle, an exposure apparatus includes a light source, a light measuring device, and a stage on which the wafer is supported. Such exposure apparatus for use in manufacturing a semiconductor device are generally classified as a contact type, a proximity type, or a projection type of exposure apparatus. Projection-type exposure apparatus may further be classified as those comprising an aligner, a stepper, or a scanner. The stepper and scanner each employ a reduction projection type of exposure system, and an aligner employs an iso-magnification projection type of exposure system.
All exposure apparatus must provide a high degree of resolution, and their optics must be able to precisely focus a pattern of light onto a wafer. To this end, a stepper emits light onto a wafer through a reticle while the wafer is stationary. On the other hand, a scanner performs an exposure process while a reticle and a wafer are moving at a uniform speed. Furthermore, the stepper uses refraction optics, whereas the scanner uses reflection optics and a slit. Also, the useful area of the lens in a scanner is very small compared to that of the lens of a stepper. Thus, a scanner is less influenced by aberrations of the lens than a stepper, and a larger numerical aperture (NA) is available in a scanner. Still further, the exposure area is larger in a scanner, which facilitates the reproducibility of the exposure process.
Hence, the current trend is to move away from using a stepper. Instead, exposure apparatus comprising scanners have become more widely used. As chips become larger and semiconductor devices become more highly integrated, the next generation of exposure apparatus will be able to expose wafers over even larger fields.
Nonetheless, an improper calibration of an exposure apparatus or a rapid change in scanning direction may cause a defocus to occur in the exposure process. Such a defocus problem and a conventional method to solve the problem will be described below referring to FIGS. 1A, 1B and 2.
FIG. 1A illustrates an exposure apparatus 10 for use in manufacturing a semiconductor device according to the prior art. FIG. 1B is a table of leveling data generated during an exposure process carried out by the exposure apparatus 10.
Exposure apparatus 10 includes a reticle 1, a light source 2 that emits light through the reticle 1, a reticle stage 3 on which the reticle 1 is mounted, a lens 4 by which light passing through reticle 1 is focused onto a desired portion (shot) of a wafer 5 coated with a layer of photoresist, a level detector 6 for sensing information pertaining to the position/configuration of a surface of the wafer, a wafer stage 8, and a wafer stage position controller 7 to control the position of a wafer stage 8. Leveling data for a wafer is generated by the level sensor 6 during an exposure process, and is discarded after the exposure process.
In FIG. 1B, the squares represent respective shots on a wafer, and the data within each square includes the leveling data measured during the exposure process for the respective shot. Reference numeral 11 indicates data identifying the shot by number. Reference numeral 12 indicates data representative of the relative height of a surface of the wafer for the respective shot. Reference 13 indicates data representative of the degree to which the surface is inclined relative to the horizontal for the respective shot, and reference numeral 14 indicates data representative of the progression of the scan in forward or reverse directions (up or down directions in the figure)and a value representing the degree to which the lens 4 was out of focus prior to exposing the shot (focus error).
That is, to scan the pattern of a reticle onto a respective portion (shot) of the wafer 5, the wafer 5 is scanned by moving the lens 4 in one of two opposite scanning directions referred to hereinafter as downward and upward directions. A scan by the lens 4 in one of the directions will be referred to as a down scan, and a scan by the lens 4 in the other of the directions will be referred to as an up scan. Furthermore, a shot exposed using a down scan of the lens will be referred to as a down scan shot, and a shot exposed using an up scan of the lens 4 will be referred to as an up scan shot. In FIG. 1B reference “U” denotes an up scan shot, and reference “D” denotes a down scan shot.
A critical dimension (CD) of the pattern formed on the wafer 5 is measured to determine whether the circuit pattern on reticle 1 has been properly transferred to the wafer 5. If the pattern formed on the wafer 6 has a CD outside of a desired range, the lens 4 is checked to determine whether it has drifted out of focus.
More specifically, and again referring to FIGS. 1A and 1B, the lens 4 may drift out of focus between an up scan and a down scan. However, it is difficult to determine from the leveling data whether the lens 4 is out of focus. Furthermore, as was mentioned above, the leveling data is temporarily stored in the exposure apparatus 10 and is then discarded. Therefore, the exposure process must be stopped, and a specific leveling qualification test (LQT) must be performed using the leveling data to determine whether the lens 4 is out of focus.
FIG. 2 is a flowchart illustrating an LQT using the leveling data shown in FIG. 1B.
With reference to FIG. 2, a critical dimension (CD) measuring device checks whether the CD of an actual pattern formed on a wafer exceeds the desired or design CD (S21). If the measured CD is acceptable, an exposure process is performed (S27) on the next wafer. However, if the measured CD exceeds a certain value, the exposure process is stopped (S22), and then an LQT using leveling data is executed (S23). The result of the LQT, i.e., the value of the defocus, is outputted to a display device (S24). At this time, if the defocus exceeds a threshold value (S25), the focus of the lens 4 is re-calibrated (S26) and then, the next exposure process is performed (S27). On the other hand, if the value of the defocus is below the threshold value, the next exposure process is performed.
A disadvantage with this method is that the exposure process must be stopped to detect whether the lens is out of focus, and then the level qualification test must be performed. The stopping, the testing, and the re-starting detract from the productivity of the overall process of manufacturing the semiconductor devices.